Birds have a remarkable ability to tolerate altitude exposure and have been reported flying at altitudes where mammals become unconscious or even die from lack of oxygen. Such altitudes place extraordinary demands on the bird's cardiovascular system. Birds have larger hearts than do similar size mammals, and I hypothesize that this large heart allows birds a larger stroke volume and cardiac output. In this investigation I will measure cardiac output (using the Fick principle), stroke volume, heart rate and the rate of oxygen consumption during rest and exercise on a variety of birds having widely different body masses. Each of these parameters will be related to each other and to avain heart and body mass to provide information on the basic physiology of the avian cardiovascular system. This information will be especially valuable in comparison with similar mammalian data to determine the role of the large avian heart in cardiovascular performance. It is expected that information gained by this study will help explain the exceptional ability that birds have for vigorous exercise at high altitude, and this may give more indications as to what most limits mammaliam tolerance to high altitude. Also, the information gained may help in our understanding of the physiology of the hypertrophied human heart.